Covalent Organic Frameworks (COFs) are an emerging class of porous crystalline material, constructed via strong covalent bonds between lighter elements like C, B, O, N and Si. Due to their low density and n-n stacked architecture COFs have been used as effective gas storage media, as a catalytic support and semi conductive and photo conductive device.
Recently mechanochemistry has been efficiently employed to carry out various organic and inorganic transformations, nanostructure formation, metal-organic framework construction and thus become a good alternative to classical solution based synthesis. Accordingly the mesoscopic organic nanosheets peeled from stacked 2D covalent frameworks is reported in Chem. Commun., 2011, 47, 7365-7367 by Yugen Zhang et al. WO/2013/006623 discloses preparation of coating suspensions of MWW- and MFI-nanosheets by dissolution of the exfoliated zeolite-polymer nanocomposite in toluene followed by ultra-sonication.
There are some of the articles describing the synthesis of 2D COF such as San-Yuan Ding, in J. Am. Chem. Soc 2011, 133, 19816-19822 discloses the synthesis of imine-based COF material, COF-LZU1, comprises reaction of 1,3,5-Triformylbenzene and 1,4-diaminobenzene in presence of 1,4-dioxane and aqueous acetic acid in a liquid nitrogen bath at temperature 120° C. to yield COF-LZU1 (90% yield). Accordingly, the Pd(OAc)2 was incorporated into COF-LZU1 to form Pd/COF-LZU1 material, which was further applied to catalyze the Suzuki Miyaura coupling reaction, for the formation of C—C bonds.
“Surface-Confined Crystalline Two-Dimensional Covalent Organic Frameworks via on-Surface Schiff-Base Coupling” is disclosed in ACS Nano. 2013 Aug. 12 by Xu L, Zhou X, et al., comprising co-condensation reaction between aromatic aldehyde and aromatic diamine monomers on a highly oriented pyrolytic graphite surface either at a solid/liquid interface at room temperature or in low vacuum with moderate heating to obtain surface-confined 2D covalent organic frameworks (COFs) with few defects and almost entire surface coverage.
The article relates to Imine-Linked Porous Polymer Frameworks with High Small Gas (H2, CO2, CH4, C2H2) Uptake and CO2/N2 Selectivity” is reported by Youlong Zhu et al. in Chem. Mater., 2013, 25 (9), pp 1630-1635.
Modified mechanochemical synthesis were employed for the rapid synthesis of MOFs by using liquid assisted grinding (LAG) to enhance the topological selectivity and to construct 0 D porous organic cages. Although, mechanochemistry is one of the most suitable synthetic tool for the formation of covalent bonds but no process for the synthesis of 2D or 3D covalent organic framework (COF) materials by using mechanochemical strategy has been disclosed yet.
The fundamental requirement for the COF crystallization is the reversibility in covalent bond formation; therefore, achieving the same via mechanochemical synthesis approach is a daunting challenge. In general, harsh experimental conditions such as, reaction in a sealed pyrex tube, Inert atmosphere, choice of suitable solvents, reaction rates, longer time for crystallization etc. required during COF synthesis to form well-ordered crystalline frameworks. Moreover, once formed special care requires for storage of COF samples due to their moisture instability. Hence, an advance synthetic method like mechanochemical grinding and proper optimization of the reaction conditions needed to be explored in order to overcome these principal issues.
It is believed that reversibility in covalent bond formation during synthesis is required for the successful crystallization of COFs which is necessary to identify their specific structural details precisely. Irreversible organic reactions always lead to the formation of amorphous porous polymeric materials, separately categorized as Porous Organic Polymers (POPs) or Porous Aromatic Frameworks (PAFs). Even though most of the POPs/PAFs have high thermal and chemical stability they are amorphous in nature and do not have any internal ordering. Since reversible back reactions can occur after the synthesis, COFs in general get completely decomposed even in presence of ambient humidity. Little improvement in water stability was achieved by pyridine doping (Chem Commun 2012, 48, 4606) and alkylation of COF pore walls (J. Am. Chem. Soc. 2011, 130, 11872). However, these modifications always lead to decrease in the gas adsorption properties even though it enhances the hydrolytic stability to a small extent. Hence, stability problem in COFs still remain a challenge which prevents the usage of COFs for diverse practical applications.
There are few reports on 2D COFs grown on surfaces or synthesized by ultra-sonication, these processes are highly energy consuming and need special precautions, such as the usage of dry solvents, ultra high vacuum and also the need of expensive supports. The article titled “Delamination of Layered Covalent Organic Frameworks” by Isadora Berlanga et al. in Small Volume 7, Issue 9, pages 1207-1211, May 9, 2011 discloses the isolation of nanostructures consisting of 10-25 layers of a covalent organic framework by means of selective ultrasound exfoliation on the bulk layered material.
Although scalable production of graphene sheets by mechanical delamination has already been utilized to synthesize graphene from graphite, (Catharina Knieke et al. Carbon 48 (2010) 3196-3204) not a single effort has been made to delaminate the chemically stable COFs using mechanical solid state grinding due to instability of most COF materials under ambient conditions, which forbids the use of mechanical force.
Therefore, the construction of bonds through simple, economical and environmentally-friendly mechanochemical route to form highly stable covalent organic frameworks is highly desirable In modern synthetic chemistry.